HUS

HUS is a form of thrombotic microangiopathy caused by endothelial damage, most classically from Shiga toxin produced by enterohemorrhagic E. coli (EHEC O157:H7) or Shigella dysenteriae. The toxin damages vascular endothelium, particularly in the renal glomeruli, triggering platelet activation and fibrin deposition in small vessels. This leads to mechanical destruction of red blood cells (producing schistocytes on peripheral smear), consumption of platelets (thrombocytopenia), and renal microthrombi causing acute kidney injury.

Classic presentation: child with bloody diarrhea (from undercooked beef, unpasteurized milk, or contaminated produce) followed 5-10 days later by pallor, fatigue, oliguria, and sometimes petechiae. Labs show anemia with schistocytes, elevated LDH, low haptoglobin, thrombocytopenia, and elevated creatinine/BUN. Coombs test is negative (distinguishing from autoimmune hemolysis).

Treatment is primarily supportive—fluid and electrolyte management, dialysis if needed, and blood transfusions as required. Antibiotics are generally avoided in STEC-associated HUS as they may increase toxin release and worsen outcomes. Antimotility agents should also be avoided.

Important distinctions: - Atypical HUS: not associated with diarrhea, caused by complement dysregulation (mutations in complement regulatory proteins); treated with eculizumab (complement inhibitor). - TTP (Thrombotic Thrombocytopenic Purpura): similar pentad but includes fever and neurologic symptoms, caused by ADAMTS13 deficiency (either congenital or autoimmune), more common in adults; treated with plasma exchange.

High-yield boards association: HUS is a classic example tested with STEC/E. coli O157:H7 exposure (undercooked ground beef), and distinguishing it from TTP based on ADAMTS13 levels and clinical triad vs pentad is frequently tested.